How Many Times Do You Have To Listen To Understand A Piece Of Music?

Once.

I know it's not fair, but I am quite judgmental when listening to new music.

If I don't want to listen to the entire song, then it has almost zero chance of getting a second chance.

I don't think you have to objectively "understand" a piece of music to absolutely enjoy it right away, from complex jazz or classical to a simple tune. If it feels good, it's good. I was a Beatles and Dylan fan in the 60s when it was new and would sit stunned when first hearing some new thing...saw Monk at 13 when he opened for Peter Paul and Mary at the Hollywood Bowl...didn't know any jazz, no clue who he was, got my mind blown. All that resulted in a decades long career playing music for a living.

Rap or hip-hop: about 15 seconds.  Most contemporary pop/rock: first listen.  Selected pop artists (mostly indie, e.g. Kate Bush): a couple of listens.  Springsteen: ain't gonna happen in this lifetime.

If I’m critically listening, not hearing at a holiday party at someone house, once.  Some music grows on me like an earworm.  My kids music sometimes.  I’ve heard it so many times I know every word by heart but I wouldn’t put it on my playlist.